Add docs for rate limiter on raw endpoints

The section was included on security/raw_limits and on the admin area
settings.

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/48717

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diff --git a/doc/user/admin_area/settings/rate_limits_on_raw_endpoints.md b/doc/user/admin_area/settings/rate_limits_on_raw_endpoints.md
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+---
+type: reference
+---
+
+# Rate limits on raw endpoints **(CORE ONLY)**
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30829) in GitLab 12.2.
+
+This setting allows you to rate limit the requests to raw endpoints, defaults to `300` requests per minute. 
+It can be modified in **Admin Area > Network > Performance Optimization**.
+
+For example, requests over `300` per minute to `https://gitlab.com/gitlab-org/gitlab-ce/raw/master/app/controllers/application_controller.rb` will be blocked.
+
+![Rate limits on raw endpoints](img/rate_limits_on_raw_endpoints.png)
+
+This limit is:
+
+- Applied independently per project, per commit and per file path.
+- Not applied per IP address.
+- Active by default. To disable, set the option to `0`.